A cementitious wall is externally reinforced against out-of-plane or in-plane seismic forces using an overlying layer of composite material, preferably a carbon-fiber/polymer matrix composite material. To externally reinforce the wall against out-of-plane forces, the composite material is applied to the wall as an overlay with the fibers extending vertically. At the base of the wall, the composite overlay is tied to the horizontal building structure using right-angle ties spaced-apart along the foot of the wall. The composite overlay may either be structurally tied to intervening floor structures using right-angle ties, or it may be structurally isolated from the intervening floor structures using flat-tie pass throughs. To externally reinforce the wall against in-plane horizontal forces, the composite material is applied to the wall as an overlay with the fibers running only horizontally.